Biotinylated fluorescent ligands are being attached to beads, and studied for cytometry, for the purposed of developing biosensors. This project is described in detail in the year 16-20 NFCR renewal proposal. In brief, the project has three elements: 1) to characterize the interaction of biotiylated ligand with strepavidin beads; 2) to detect energy transfer on the bead surface between ligands and receptors; 3) to use the energy transfer between tethered fluorescent ligands and receptors to monitor the presence of soluble ligands. 1) and 2) have been accomplished. A manuscript for 1) is in preparation; a manuscript for 2) is planned. Novel results include the detection of site-site interactions between ligands on the bead surface using phase sensitive flow cytometry.